In recent years, there has been provided an LED illumination apparatus using light emitting diodes as a light source (see, e.g., Japanese Patent Application Publication No. 2009-134946). The LED illumination apparatus has an apparatus body in which an LED light emitting unit including light emitting diodes and a lighting circuit unit supplying the LED light emitting unit with a current for turning on the light emitting diodes are installed. The lighting circuit unit includes a switching power supply circuit section and a filter circuit section.
The switching power supply circuit section is a nonisolated step-down chopper circuit whose switching is controlled by a control circuit also serving as a power switching element (e.g., MIP552 manufactured by Matsushita Electric Industrial Co., Ltd.). In such LED illumination apparatus, if the ambient temperature changes and the temperature increases to a high temperature from a low temperature, the control is performed to increase the light output by increasing the current flowing through the light emitting diodes, thereby making the light output to be substantially constant.
Connected to the input terminal of the switching power supply circuit section of the above conventional example is an input capacitor which smoothes the voltage that is full-wave rectified by a full-wave rectifier. If a capacitance value of the input capacitor is small, a ripple component of the load current flowing through the LED light emitting unit increases. For example, if an image is captured by a video camera or the like, there is a possibility that flickering occurs on the screen when a shutter speed is not synchronized to the frequency of a commercial power source. The ripple component can be reduced by increasing the capacitance value of an output capacitor connected to the output terminal of the switching power supply circuit section, but a very large electrolytic capacitor is required in order to fill the valleys of the load current occurring at zero-crossing of a source voltage.
As measures to improve such a problem, there is a method to increase the capacitance value of the input capacitor by adopting an electrolytic capacitor as the input capacitor. If the voltage smoothed by the input capacitor can be maintained to be equal to or greater than a certain level even at zero-crossing of the source voltage, a constant load current can flow at all times by a constant current circuit that is connected at a subsequent stage of the switching power supply circuit section.
In other words, by increasing the capacitance value of the input capacitor, it is possible to remove the ripple component of the load current without having to increase the capacitance value of the output capacitor. However, in this case, since it becomes a so-called capacitor input type smoothing circuit, there are problems such that a conduction angle of the input current (charging current to the input capacitor) becomes narrow, power factor deteriorates into being equal to or less than 0.6, and harmonic distortion becomes greater.
Here, in order to reduce the ripple component of the output current (load current) of the switching power supply circuit section and realize the improvement of the power factor, it is general to adopt a so-called two-converter method in which a power factor correction circuit (PFC circuit) is provided at a front stage of the constant current circuit. However, in this case, there are problems such that noise increases and circuit configuration becomes complicated by adding the power factor correction circuit that is a high frequency switching circuit.
In this regard, configurations to reduce the ripple component of the load current and improve the power factor by providing, as a smoothing unit, a partial smoothing circuit for partially smoothing low voltage periods of a ripple voltage outputted from the full-wave rectifier instead of the input capacitor have been disclosed in, e.g., Japanese Patent Nos. 3263194 and 3327013. In such configurations, since the output voltage of the partial smoothing circuit is equal to or greater than a certain value even during the low voltage periods, a constant load current can flow at all times by the constant current circuit that is connected at the subsequent stage thereof, thereby removing the ripple component of the load current. Further, compared to the case of the capacitor input type smoothing circuit, it is possible to improve the power factor.
However, in the case where the partial smoothing circuit is adopted as described above, since a steep charging current flows through the capacitor constituting the partial smoothing circuit, there is a problem such that the peak value of the input current of the partial smoothing circuit increases. In this case, it is requested to use parts with a large current withstanding capability and, accordingly, it is required to increase the thickness of the wire or increase the capacity of a switch or breaker, which causes an increase in cost or size of the apparatus.